This invention relates to ultrasonic transducers and their method of fabrication and more particularly to an improved technique for bonding the layer of piezoelectric material to its supporting layer and to its impedance matching layer.
Typically, a prior art transducer 16 of the lead-zirconate-titanate (PZT) type suitable for use in medical ultrasonics has been fabricated as shown in FIG. 1. A layer 10 of PZT has silver electrodes 11 deposited on both sides of the PZT layer 10. Electrical leads 12 are connected to the silver electrodes 11 in order to allow electrical energization of the piezoelectric material. A glass layer 13 is bonded by an epoxy layer 14 to the silver electrode 11 on the upper side of the PZT layer 10 in order to provide acoustic impedance matching between the PZT layer 10 and the water of the container in which the transducer 16 is placed when it is being used. A supporting layer 15 is attached by the epoxy layer 14 to-the silver electrode 11 on the lower side of the PZT layer 10. The supporting layer 15 typically of lead, serves to broaden the bandwidth and to absorb energy generated by the layer 10 and thus prevent reflections within the transducer. The epoxy layers 14, 16 are desired to be as thin as possible to minimize distortion of the acoustic wave passing through the layers 14, 16, such distortions being significant even for epoxy layers having thicknesses of approximately one half mil.
A transducer array 17 is comprised of individual transducer elements 18, each of which includes the layer of glass 13 and PZT piezoelectric layer 11, both mounted to a supporting layer 15. The layer of glass 13, the PZT piezoelectric layer 10 and the supporting layer 15 are initially epoxy bonded to each other to form a large area transducer after which a diamond saw is used to cut slits 19 through the glass layer 13 and the PZT piezoelectric layer 10 to form the individual radiating elements 18 of a linear transducer array 17, as shown. Typically, each one of the transducer elements 18 has a length of 8 mm and a width 20 of 2.5 mm. The elements 18 are spaced from each other by the width of the saw cut of slot 19 of approximately 0.1 mm.
It has been found that the structural reliability of the prior art transducer array fabricated as described is rather poor with bond failure often occurring especially at the PZT/glass interface. The failure often occurs during the sawing operation while forming the elements 18, or shortly thereafter when the transducer is being operated in its normal water environment.
It is believed that the poor reliability obtained with an epoxy bond is a result of a number of factors. Lateral cracking at the edges of the elements 18 is produced during the sawing operation. The effect of this cracking becomes more severe as the width 20 of the element decreases to 1-3 mm as desired in the transducer of this invention. The brittle nature of the epoxy bond 14 leads to further stress concentration effects. Further, it is a well established fact that the bond strength is often seriously degraded when subject to stress in a water environment in which the transducer to which the invention is directed is commonly employed.